1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include one or more base stations, which may also be referred to as access points or node-Bs or access networks, for providing wireless connectivity to one or more mobile units, which may also be referred to using terms such as user equipment, subscriber equipment, and access terminals. Exemplary mobile units include cellular telephones, personal data assistants, smart phones, text messaging devices, laptop/notebook computers, desktop computers, and the like. Each base station may provide wireless connectivity to one or more mobile units, such as the mobile units in a geographical area, or cell, associated with the base station. For example, a base station may provide wireless connectivity to mobile units located in a cell according to a Universal Mobile Telecommunication System (UMTS) protocol. Alternatively, the base station may provide wireless connectivity to mobile units located in the cell according to a CDMA protocol, an IEEE 802.11 protocol, an IEEE 802.16 protocol, an IEEE 802.20 protocol, a Bluetooth protocol, and or other protocols.
In operation, each mobile unit maintains a wireless communication link, or leg, with one or more base stations. The wireless communication link typically includes one or more uplink and/or downlink channels for transmitting data and control information. Speech traffic is conventionally transmitted over the wireless communication link using circuit-switched radio bearers so that a permanent connection exists between the mobile unit and the base station and/or radio access network. However, procedures to improve the efficiency of the usage of scarce radio resources are being implemented in advanced speech coding schemes, such as Adaptive Multi-Rate (AMR) techniques. Channelization codes that are used to define the uplink and/or downlink channels are among the most restrictive resources in a wireless telecommunication System. For example, in UMTS systems, the channelization codes may be Walsh codes with a spreading factor of 128, in which case at most 128 concurrent connections can be supported by a single base station in one cell. Furthermore, in real systems the number of concurrent connections is typically reduced by interference limits, soft handover overhead, and the like.
Voice over Internet Protocol (VoIP) systems transmit data and/or control information using a packet-switched network, such as the Internet. In VoIP systems, speech data frames are embedded into Internet Protocol data frames. Two different types of packets are typically transmitted: Real Time Protocol (RTP) packets that carry the voice payload and Real Time Control Protocol (RTCP) packets that carry control information such as synchronization information, information that may be used for conference calls between multiple parties, and the like. Although the RTP packets and the RTCP packets may be multiplexed at the Internet Protocol level, the quality-of-service requirements of the two types of packets are different. The RTP packets are subject to relatively stringent delay constraints (i.e., they are delay-intolerant) but they are relatively tolerant to loss. In contrast, the RTCP packets are delay tolerant but are very sensitive to loss. Furthermore, the RTP packets are generally transmitted frequently and/or periodically and have predictable sizes, whereas the RTCP packets may be transmitted occasionally and/or sporadically but they have larger and/or varying sizes.
The Third Generation Partnership Project (3GPP) has attempted to define mechanisms that may be used to distinguish between the RTP and RTCP packet flows so that the two packet flows may be assigned to different radio bearers that take into account their different properties. In particular, different operational modes of the Radio Link Control (RLC) protocol layer may be used: Unacknowledged Mode (UM) RLC, which provides low delay jitter at the cost of increased residual packet loss rate may be used for the RTP flow, and Acknowledged Mode (AM) RLC, which provides lower residual packet loss rates at the cost of higher delay jitter may be used for the RTCP flow. However, no acceptable technique for distinguishing between the two packet flows has yet been proposed. Two proposed mechanisms for distinguishing between the RTP and RTCP packet flows—completely removing RTCP packets and using User Datagram Protocol (UDP) port numbers or packet-type header fields to distinguish between the flows—are considered unreliable and have been rejected by the 3GPP.